The present invention relates to a vehicle speed responsive power steering assembly and particularly to a power steering assembly in which the amount of power assist is varied in accordance with vehicle speed.
Vehicle speed responsive power steering assemblies in which the amount of power assist is varied by varying the displacement of a valve member of a control valve which controls the flow of pressurized oil to fluid chambers of a power steering actuator, are known. One of such assemblies is shown in FIG. 12. The assembly shown in FIG. 12 comprises a power steering pump 1 and a control valve 3 that controls oil flow to a power assist actuator 4. A conduit communicating the pump 1 with the control valve 3 has a restriction orifice 2 therein. Return oil from the actuator 4 passes through the control valve 3 back to a reservoir 5. The control valve 3 includes a restricting mechanism 7 including a piston 6. The pressurized oil from the pump 1 also passes through a changeover valve 8 to a fluid chamber of the restricting mechanism 7 and through a orifice 9 to the inlet of a vehicle speed responsive pump 10. The outlet of the pump 10 is connected to the reservoir 5. The upstream side of the orifice 2 is connected through the changeover valve 8 with the fluid chamber of the restricting mechanism 7 and the orifice 9. The changeover valve 8 is controlled by a pressure generated by the pump 10 which rotates at vehicle speed. When the vehicle speed increases, the changeover valve 8 operates to communicate the oil pressure upstream of the orifice 2 to the fluid chamber of the restricting mechanism 7 which moves the valve body of the valve 3 to a neutral position of the valve 3.
Another known vehicle speed responsive power steering assembly is shown in FIG. 13. The assembly shown in FIG. 13 also comprises a power steering pump 1, a restriction orifice 2, a control valve 3, a power assist actuator 4, a pump reservoir 5, a restricting mechanism 7 including a piston 6, and a changeover valve 8 which are essentially similar to those shown in the assembly of FIG. 12. As shown in FIG. 13, a flow-rate control valve 12 is connected through a pressure control valve 11 with the pump 1. The pressure control valve 11 is connected with the outlet of the flow-rate control valve 12 and the restricting mechanism 7. Solenoid 13 of the flow rate control valve 12 is connected with a control device 14 to which a signal from a vehicle speed sensor 14' is applied.
In the assembly of FIG. 13, a vehicle speed signal from the vehicle speed detector 14' is applied to the control device 14 which generates a signal applied to the solenoid 13 of the flow-rate control valve 12 which controls oil flow through the changeover valve 8 and pressure control valve 11 which communicates oil pressure upstream of the orifice 2 to the fluid chamber of the restricting mechanism 7.
The known assemblies shown in FIGS. 12 and 13 have complex structure, necessitate high accuracy, and are expensive to manufacture. Especially, the device of FIG. 13 includes a plurality of control and changeover valves so that the reliability of the assembly decreases. Because restriction of oil pressure is required, while running, to increase primary fluid energy of the power pump, a cooling mechanism is necessary to prevent the operating oil from overheating.